You have a fair point, but that's nothing more than a presumption (unless I've missed something key). My train of thought is, I presume the lack of a clause to be almost exclusive to fee originally agreed & would be a more probable reason as to why there wasn't one in this case.
Recalled Kenedy & Gallagher/Gilmour had clauses depending on a games played quota. The lack of any concrete information, from what I've read, results in theories based on presumptions.